Information Technology Reference
In-Depth Information
code it? If a file could be interpreted in several ways, perhaps
as text or as a number or as an image, which interpretation
should the filter use?
Deciding What to Block: Any software filter is a computer pro
gram, and programs follow algorithms; therefore, any software fil
ter must have a set of instructions or rules for determining which
materials can pass and which must be blocked. To provide flexibil
ity, the filters may offer a range of rules that the user or administra
tor can establish for her or his computer. Regardless of the specific
technology, however, at some level the software must depend upon
rules to decide its actions. These rules may follow several ap
proaches, such as the following:
A filter may contain a table of Web sites that are known to
have appropriate material; the filter passes material from sites
on the list, but sites not listed in the table are blocked.
A filter may contain a table of sites known to have objection
able material; the sites listed are blocked, but others are as
sumed acceptable.
A filter may search for certain “objectionable” words or
phrases and block pages in which those are found.
A filter may rely upon keywords or other information pre
pared by the authors of pages. Those pages with acceptable
keys are allowed to pass, but others are blocked.
A filter may rely upon keywords or other authorobtained in
formation, and the keywords may be used to identify “objec
tionable” pages.
Unfortunately, none of these approaches are very effective in
blocking inappropriate materials while allowing appropriate mate
rials to pass. For example, relying on a table of acceptable Web sites
certainly blocks much objectionable material. However, with the
dramatic and ongoing growth of the Web, it takes time for new rea
sonable sites to be added to the filter's list, and such sites would be
blocked—at least for some time. As a particular example, for awhile
one wellknown filtering package allowed material from the
“National Republican Party” to pass, but blocked material from the
“National Democratic Party.” According to the vendor, someone
